For the past 20 years scientists have been fretting over the decline of sperm counts in the West. The most recent alarm came late last year, when a study found that sperm counts had fallen in French men from an average in 1989 of 73.6 million sperm per milliliter of semen in a 35-year-old man to 49.9 million per milliliter in 2005.

While nowhere close to threatening our fertility as a species—that number would need to drop below 15 million per milliliter—it is not a comforting trend given that lower sperm counts make it harder to father children. But the cause has been unknown. Did the lower sperm counts stem from high-fat diets, being overweight, or trace amounts of chemicals in the environment and their effect on the body’s hormones? The search for a smoking gun produced a lot of intense speculation but little firm evidence.

Now, a deceptively simple study published in theBritish Journal of Sports Medicine has found a new and surprising culprit: television.

“Men who watched more than 20 hours of TV every week had 44 percent lower sperm counts compared to those who watched almost no TV,” says lead author Audrey Gaskins, a doctoral candidate at Harvard’s School of Public Health. “And then men who exercised for 15 hours or more per week at moderate to vigorous levels had 73 percent higher sperm counts than those who exercised less than five hours per week.” Read full article.